Native antibodies and immunoglobulins are usually heterotetrameric glycoproteins of about 150,000 daltons, composed of two identical light (L) chains and two identical heavy (H) chains. Each light chain is linked to a heavy chain by one covalent disulfide bond, while the number of disulfide linkages varies between the heavy chains of different immunoglobulin isotypes. Each light chain is comprised of a light chain variable region (abbreviated herein as VL) and a light chain constant region (abbreviated herein as CL). Each heavy chain is comprised of a heavy chain variable region (VH) and a heavy chain constant region (CH) consisting of three domain, CH1, CH2 and CH3). CH1 and CH2 of the heavy chain are separated from each other by the socalled hinge region. The hinge region normally comprises one or more cysteine residues, which may form disulphide bridges with the cysteine residues of the hinge region of the other heavy chain in the antibody molecule.
Recently, antibodies have become a major focus area for therapeutic applications, and many antibody drug products have been approved or are in the process of being approved for use as therapeutic drugs. The desired characteristics of therapeutic antibodies may vary according to the specific condition which is to be treated. For some indications, only antigen binding is required, for instance where the therapeutic effect of the antibody is to block interaction between the antigen and one or more specific molecules otherwise capable of binding to the antigen. For such indications, the use of Fab fragments, the only function of which is to bind antigen, may be preferred. For other indications, further effects may also be required, such as for instance the ability to induce complement activation and/or the ability to for instance bind Fc receptors, protect from catabolism, recruit immune cells, etc. For such use, other parts of the antibody molecule, such as the Fc region, may be required. Some full-length antibodies may exhibit agonistic effects (which may be considered to be undesirable) upon binding to the target antigen, even though the antibody works as an antagonist when used as a Fab fragment. In some instances, this effect may be attributed to “cross-linking” of the bivalent antibodies, which in turn promotes target dimerization, which may lead to activation, especially when the target is a receptor. In the case of soluble antigens, dimerization may form undesirable immune complexes.
In some cases, monovalent binding to an antigen, such as in the case of FcαRI may induce apoptotic signals (Kanamura et al, Blood published on line Sep. 25, 2006))
For some indication, monovalent antibodies may thus be preferable. The presently available Fab fragments show inferior pharmacokinetics due to their small size resulting to filtration in the kidneys as well as their inability to interact with the Brambell receptor FcRn (Junghans R P et al., Proc Natl Acad Sci USA 93(11), 5512-6 (1996)), therefore being unstable in vivo and having very rapid clearance after administration.
Dimeric, monovalent antibodies (Fab/c), wherein the Fc region comprises two Fc polypeptides, have also been described (WO200563816 to Genentech and Parham P, J. Immunol. 131(6), 2895-902 (1983).
There is thus a need for stable monovalent antibodies for use as therapeutics.
Deletion of one or more of the domains of full-length antibodies, covering for instance regions comprising amino acid residues necessary for forming di-sulphide bridges or providing non-covalent inter-heavy chain contacts in the antibody may be a way of constructing monovalent antibodies.
Igarashi et al. (Igarashi, T M. et al., Biochemistry 29, 5727 (1990)) have described the structure of a mouse IgG2a molecule in which the entire CH1 domain was deleted, but the hinge region was intact. The CH1 deleted antibody is shown to exist as an elongated structure with a relatively small hinge angle. The molecule however retained the regular tetrameric configuration consisting of two light chains and two heavy chains expected for IgGs, and was thus still bivalent, and the CH1 deletion did not affect the affinity of the mutated antibody.
Larson et al. (Larson, S B. et al., J Mol Biol 348, 1177 (2005)) have described the structure of a humanized IgG1 antibody in which the CH2 domain has been deleted. Such antibody exists in two molecular forms, termed form A and form B. Form A contains two inter-chain disulphide bonds in the hinge, whereas form B does not contain inter-chain disulphide bonds. Form B exists as ˜122 kDa molecule which seems to be held together by non-covalent interactions within the CH3 domain. The antibody displays rapid serum clearance because of an inability to bind and recycle through FcRn receptors.
Ig half-molecules, which have a dimeric configuration consisting of only one light chain and only one heavy chain, have been described as the result of rare deletions in human and murine plasmacytomas. Several patients suffering from extramedullary soft-tissue plasmacytoma, Waldenström macroglobulinemia, plasma cell leukemia and multiple myeloma, excreted IgG half molecules into their urine. Half-molecules were also found to be present in their serum. Studies on the biochemical nature of these half-molecules showed that they consist of IgG1 molecules in which the heavy chain CH1, hinge and CH2 regions appeared normal, whereas deletions were found in the CH3 region. The deletion on the CH3 constant domain in the IgG1 half-molecule analyzed by Spiegelberg was shown to encompass 5,000-8,000 dalton and the hinge peptide sequence was identical to wild type IgG1. The mutations appeared to be located in CH3 and the hinge peptide appeared normal (Hobbs, J R et al., Clin Exp Immunol 5, 199 (1969); Hobbs, J R, Br Med J 2, 67 (1971); Spiegelberg, H L et al., Blood 45, 305 (1975); Spiegelberg, H L et al., Biochemistry 14, 2157 (1975); Seligmann M E et al., Ann Immunol (Paris) 129C, 855-870 (1978); Gallango, M L et al., Blut 48, 91 (1983)). It was also showed that this human IgG1 half-molecule is rapidly catabolized (half-life in man was 4.3 days) and, in monomeric form, is unable to bind C1q or Fc receptors on human lymphocytes, monocytes or neutrophils (Spiegelberg, H L. J Clin Invest 56, 588 (1975)). It was concluded from these studies that the IgG1 half-molecule lacks non-covalent interactions characteristic for the Fc portion of the IgG heavy chain which destabilizes the molecule, and that the CH3 domain may be particularly important in maintaining the interactions between IgG heavy chains.
Murine IgA half-molecules which were generated by somatic mutation have also been described (Mushinski, J F, J Immunol 106, 41 (1971); Mushinski, J F et al., J Immunol 117, 1668 (1976); Potter, M et al., J Mol Biol 93, 537 (1964); Robinson, E A et al., J Biol Chem 249, 6605 (1974); Zack, D J et al., J Exp Med 154, 1554 (1981)). These molecules were shown to all contain deletions of the CH3 domain or mutations at the CH2-CH3 boundary. Human IgA half-molecules have also been detected in patients with multiple myeloma. These molecules were found to have deletions located to the CH3 regions as well (Spiegelberg, H L et al., J Clin Invest 58, 1259 (1976); Kawai, T. et al., Ann Acad Med Singapore 9, 50 (1980); Sakurabayashi, I. et al., Blood 53, 269 (1979); Biewenga, J. et al., Clin Exp Immunol 51, 395 (1983)).
Human IgG1 mutants with hinge deletions have been described and crystallized (Saphire, E O. et al., J Mol Biol 319, 95 (2002)). Dob and Mcg are human myeloma proteins of the human IgG1 subclass which contain a deletion of the hinge region. These hinge deleted IgG1 molecules form stable Igs with a structure consisting of two heavy and two light chains, which is the typical heterotetrameric structure of antibodies, that however form inter-chain disulphide bonds between the light chains resulting in molecules that are strongly conformationally restricted and which display little to no effector function (Burton D R et al., J Mol Biol 319, 9 (2002); Steiner, A et al., Biochemistry 18, 4068 (1979); Silverton, E W et al., Proc Natl Acad Sci USA 74, 5140 (1977); Rajan, S S et al., Mol Immunol 20 787 (1983); Guddat, W et al. Proc Natl Acad Sci USA 90, 4271 (1993); Sarma, R. et al., J. Applied Cryst. 15, 476 (1982); Klein, M., et al., Proc Natl Acad Sci USA 78, 524 (1981)).
An IgG3 molecule in which the upper and middle hinge regions or the full hinge region was deleted, has been designed (Brekke, O H et al., Nature 363, 628 (1993); Brekke, O H et al., Nature 383, 103 (1996)). The molecule with the complete hinge deleted showed the presence of half-molecules upon analysis on non-reducing SDS-PAGE. A second hinge deleted molecule in which the complete upper and lower IgG3 hinge were replaced by a single cysteine and the lower IgG3 hinge contained a single Ala deletion, also contained half-molecules when analyzed on SDS-PAGE. However, the results show that under physiological conditions, the two heavy-light chain half-molecules are held together by non-covalent interactions between the IgG3 CH3 domains; and intact IgG molecules were therefore formed.
A matched set of chimeric IgG1 and IgG4 antibodies has also been prepared (Horgan, C. et al. J Immunol 150, 5400 (1993)). To investigate the role of the IgG hinge region in antibody binding to antigen, mutants were prepared of both IgG1 and IgG4 which lacked the hinge region. The mutants were generated at the DNA level by deleting the hinge region exon from the IgG1 and IgG4 heavy chain genes. It was reported that both the IgG1 and IgG4 hinge-deleted molecules were bivalent, therefore having the typical heterotetrameric structure. In support of this, the functional affinity of the hinge-deleted IgG4 showed better binding to antigen than the wild-type IgG4, indicating that the avidity of the hinge-deleted molecule is not affected by the hinge deletion thus generated.
Human IgG4 molecules exist in various molecular forms which differ by the absence or presence of inter-heavy chain disulphide bonds located in the hinge region. Thus IgG4 molecules exist in which two, one or no inter-heavy chain disulphide bonds have been formed (Schuurman, J. et al., Mol Immunol 38, 1 (2001)). Under physiological conditions, these molecular forms of IgG4 may be in equilibrium with each other. Human IgG4s exist as tetramers in solution consisting of two Ig heavy and two light chains, as common for immunoglobulin G molecules, irrespective of the absence or presence of these interchain disulphide bonds (Schuurman 2001 supra; Gregory, L. et al. Mol Immunol 24, 821 (1987)). Only upon denaturation under non-reducing conditions, the two non-covalently associated half-molecules dissociate as demonstrated by size-determination analysis such as SDS-PAGE (Schuurman, J. et al. Mol Immunol 38, 1 (2001); Deng, L. et al. Biotechnol Appl Biochem 40, 261 (2004)). It has been shown that mutation of the residues of the hinge region which are involved in inter-chain disulphide bond formation or deletion of the hinge region lead to creation of a homogeneous pool of IgG4 molecules in solution, which pool consists of tetrameric molecules consisting of two light chains and two heavy chains (Schuurman, J. et al. Mol Immunol 38, 1 (2001); Horgan, C. et al. J Immunol 150, 5400 (1993)). The IgG4 hinge-deleted and mutated antibodies also demonstrated an improved capability of antigen crosslinking when compared to native IgG4 molecules (Horgan, C. (1993) supra).
A number of studies have now shown that mutation or deletion of the IgG constant region domains CH1 and CH2 do not affect the assembly of IgG molecules into their natural two heavy and two light chain heterotetrameric configuration. Recombinant antibody molecules containing different deletions in their constant regions of the heavy chain have been shown to be affected in their effector function, e.g. they are not capable of complement activating, however, they remain their ability of antigen crosslinking. Further, it has been demonstrated that antibody half-molecules containing one heavy chain and one light chain are not stable in vivo and/or have a decreased half-life in vivo. Deletions in/of the CH3 region provides half-molecules having a rapid metabolization making them unfit for most therapeutic purposes.
There is thus a need for a simple procedure for the production of a stable monovalent antibody, which would be suitable for therapeutic applications, wherein blocking of an antigen-mediated activity requires monovalent antibody binding (absence of cross-linking).